High temperature chromium-tungstenmolybdenum alloy



United States Patent Delaware No Drawing. Filed Apr. 14, 1959, Ser. No.806,384 4 Claims. (Cl. 75-176) This invention relates to achromium-tungsten-molybdenum alloy having outstanding oxidationresistance at elevated temperatures. It pertains particularly to arefractory metal alloy of this type which is designed for buckets andguide vanes of gas turbine engines in which metal temperatures reach2000 F.

The nickel base alloy and cobalt base alloy blades commonly used todayin gas turbine engines for aircraft normally have maximum servicetemperatures of approximately 1800 F. to 1900 F. This limitationnecessarily restricts the performance and efficiency of these engines.Refractory metals, such as niobium, tungsten, molybdenum and chromium,have satisfactory high melting temperatures and sufficient potentialavailability to warrant investigation as high temperature turbine bladematerials. However, each of these metals exhibits poor oxidationresistance at temperatures of 2000 F. or above. Therefore, such metalsare unsatisfactory for use in turbine blades which necessarily areexposed to extremely hot oxidizing gases. During recent years attemptshave been made to correct this deficiency by adding small amounts ofVarious alloying elements to these refractory base metals. However,these attempts have been unsuccessful since the resultant products stilldid not possess adequate oxidation resistance at the very hightemperatures under consideration.

Accordingly, a principal object of the present invention is to provide arefractory alloy which can be employed as a turbine blade material attemperatures up to 2000 F. because of its outstanding oxidationresistance at such temperatures, coupled with good hot strength andother necessary physical properties. It is considered desirable that analloy to be used for gas turbine service in air at 2000 F. have a100-hour stress-rupture life at that temperature with at least a 15,000psi. load. Such an alloy also should possess adequate fabricability anda melting point of at least 3000 F. It is preferable that turbine bladesformed of this alloy have an oxide scale thickness of not more than0.005 inch after 100 hours exposure in air at a temperature of 2000 F.

In accordance with the present invention, we have found that arefractory alloy comprising about 37% to 70% chromium, to 50% tungstenand 10% to 30% molybdenum satisfies the foregoing requirements to anoutstanding degree. A chromium content of 45% to 65% appears to providemost advantageous results, particularly with respect to the combinationof oxidation resistance and hot strength. The preferred ranges for thetungsten and molybdenum contents are to 30% and 15% to 25%,respectively. An alloy composed of 60% chromium, tungsten and 20%molybdenum, for example, has an oxide scale of less than 0.001 inch inthickness after 100 hours cyclic exposure in air at a temperature of2000 F.

Small but effective amounts of readily oxidizable me- -F. at 16 hoursexposure at temperature.

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tals, such as calcium, arsenic, misch metal, thorium, bismuth, silicon,zirconium, tantalum, aluminum and boron, may be added to the ternaryalloy to further improve its extreme high temperature properties. Ingeneral, we have found these addition agents beneficial with respect tooptimum physical properties in the following approximate maximumamounts: 0.3% calcium, 0.08% arsenic, 0.15% misch metal, 0.2% thorium,0.3% silicon, 0.05% zirconium, 0.1% tantalum, 0.5% aluminum, 0.05%bismuth, and 0.5% boron. Small quantities of various other elments,usually less than 1%, can be tolerated in thechromium-tungsten-molybdenum alloy without affecting its physicalproperties.

Photomicrographs at 500 magnifications of our newchromium-tungsten-molybdenum alloy show a small amount of needle-likephase after exposure in air at a temperature of 2400 F. This needle-likephase has been tentatively identified as a nitrogen-rich phase which isnot encountered if the metal is exposed to the same temperature in apurified argon atmosphere. Alloys containing 60% chromium, with calciumand misch metal added, appear less susceptible to this form ofcontamination than the lower chromium materials. In fact, alloys of thistype did not show any metallographic evidence of contamination after 100hours at a temperature of 2000 F., although a small amount of thisneedle-like phase was found when heated in air for 16 hours at atemperature of 225 0 F. It was further found that pre-oxidation of thechromium-tungsten-molybdenum alloy containing 0.02% calcium and 0.15misch metal in pure oxygen for 30 minutes at 1975 F. increased theminimum temperature at which this phase was detected from 2250 F. to2400 Evidently the oxidizing nitrogen-free atmosphere prevented initialN contamination and thus inhibited diffusion of nitrogen through theoxide layer.

When non-metallic inclusions are present in the grain boundaries of thealloy of this invention, de-oxidizing additions may be beneficiallymade. These integranular impurities appear to be reduced to the greatestextent by the addition of 0.05 titanium and 0.1% to 0.3% calcium.

Our tests have shown that excellent results are produced when 0.001% to0.02% calcium and 0.03% to 0.15% misch metal are added in combination toa 60% chromium-20% tungsten-20% molybdenum alloy. The average thicknessof the oxide scale on these alloys containing addition agents was notgreater than approximately 0.001 inch after 100 hours exposure in air ata temperature of 2000 -F. Similar results were also obtained when theabove-described small additions were made to various alloys comprising37% to chromium, 10% to 50% tungsten and 10% to 30% molybdenum.

The following table lists the approximate chemical compositions ofspecific examples of the chromium-tungstenmolybdenum alloy of thisinvention and shows the thick ness of the total oxide scale formed byheating in air for hours at 2000 F. In each instance the balance of thebase alloy composition is substantially all chromium.

Each of the alloys listed in the table has a melting point materially inexcess of the 3000 F. desired minimum heretofore mentioned. These alloyswere prepared by non-consumable arc-melting in an inert atmosphere ofargon plus helium. Raw materials of maximum available purity were used.The constituents of the alloys may be added either simultaneously orsuccessively.

Base Alloy in Percent Elements Added (Percent) Total (Balance OxideChromium Thickness in Inches Mo W Ca As MM Th Bi Si Zr Ta Al B o 1 Misehmetal.

Sound extrus1ons may be obtained w1th the chrom1umcons1st1ng essent1allyof about to 65% chromlum,

tungsten-molybdenum alloy of this invention by first hot pressing it'toapproximately 10% reduction in thickness at a temperature of 3 15 0 F.Initial hot working produces a wrought recrystallized structure Whichenables the alloy to be further hot Worked, such as by extrusion.Previously, it had been widely believed that a high molybdenum contentalloy containing more than about 25 chromium lacked suitable hightemperature oxidation resistance and could not be Worked Withoutcracking. As indicated above, however, the alloy described herein hasexcellent oxidation resistance and can be suitably hot Worked.

Stress-rupture tests were conductod on as-castchromium-tungsten-molybdenum alloy test bars in an argon atmosphere at atemperature of 2000 F. under progressively increasing loads from 15,000p.s.i. to 35,000 p.s.i. For example, an alloy consisting of about 60%chromium, 20% tungsten and 20% molybdenum with 0.02% calcium plus 0.15%misch metal added had a hot strength estimated in excess of 25,000p.s.i. for 100 hours life at 2000 F. The results of this specific testwere as follows.

Stress-rupture test at 2000 F. in argon Hours previous to Stress,p.s.i.: next higher stress 15,000 154.6

1 Failed.

While our invention has been described by means of certain specificexamples, it is to be understood that the scope of our invention is notto be limited thereby except as defined in the following claims.

We claim:

1. An alloy having an average surface oxide scale thickness not inexcess of about 0.001 inch after 100 hours exposure to air at atemperature of 2000 F. and having a stress-rupture life of more than 100hours under a 15,000 p.s.i. load at a temperature of 2000 F., said alloy15% to 30% tungsten, 15% to 25% molybdenum, and a small amount eifectiveto materially increase the hightemperature oxidation resistance of saidalloy of at least one readily oxidizable metal selected from the groupconsisting of calcium, arsenic, misch metal, thorium, bismuth, silicon,zirconium, tantalum, aluminum and boron.

2. A gas turbine blade having an average surface oxide scale thicknessnot in excess of about 0.001 inch after hours exposure to air at atemperature of 2000 F. and having a stress-rupture life of more than 100hours under a 15,000 p.s.i. load at a temperature of 2000 F., said bladebeing formed of an alloy consisting essentially of about 45% to 65%chromium, 15% to 30% tungsten, 15 to 25 molybdenum, and a small amounteffective to materially increase the high-temperature oxidationresistance of said alloy of at least one readily oxidizable metalselected from the group consisting of calcium not in excess of about0.3%, arsenic not in excess of about 0.08%, misch metal not in excess ofabout 0.15%, thorium not in excess of about 0.2%, bismuth not in excessof about 0.05%, silicon not in excess of about 0.3%, zirconium not inexcess of about 0.05%, tantalum not in excess of about 0.1%, aluminumnot in excess of about 0.5% and boron not in excess of about 0.5%.

3. A highly oxidation-resistant alloy having an average surface oxidescale thickness not in excess of about 0.001 inch after 100 hoursexposure to air at a temperature of 2000 F. and having a stress-rupturelife of more than 100 hours under a 15,000 p.s.i. loadat a temperatureof 2000 F., said alloy consisting essentially of 45 to 65% chromium, 15to 30% tungsten, 15 to 25 molybdenum, calcium not in excess of 0.3% andmisch metal not in excess of about 0.15%.

4. An alloy having an average surface oxide scale thickness not inexcess of about 0.001 inch after 100 hours exposure to air at atemperature of 2000 F. and having a stress-rupture life of more than 100hours under a 15,000 p.s.i. load at a temperature of 2000 F., said alloyconsisting essentially of about 15% to 30% tungsten, 15% to 25%molybdenum and the balance substantially all chromium.

(References on following page} References Cited by the Examiner UNITEDSTATES PATENTS Honda 75-176 Kurtz 75-176 Wainer 75176 5 Wlodek et a1.75-176 Buck 75176 NAHAM MARMELSTEIN, RAY K. WINDHAM,

Examiners.

1. AN ALLOY HAVING AN AVERAGE SURFACE OSICE SCALE THICKNESS NOT INEXCESS OF ABOUT 0.001 INCH AFTER 100 HOUSR EXPOSURE TO AIR AT ATEMPERATURE OF 2000*F. AND HAVING A STRESS-RUPTURE LIFE OF MORE THAN 100HOURS UNDER A 15,000 P.S.I. LOAD AT A TEMPERATURE OF 2000*F., SAID ALLOYCONSISTING ESSENTAILLY OF ABOUT 45% TO 65% CHROMIUM, 15% TO 30%TUNGSTEN, 15% TO 25% MOLYBDENUM, AND A SMALL AMOUNT EFFECTIVE TOMATERAILLY INCREASE THE HIGHTEMPERATURE OXIDATION RESISTANCE OF SAIDALLOY OF AT LEAST ONE READILY OXIDIZABLE METAL SELECTED FROM THE GROUPCONSISTING OF CALCIUM, ARSENIC, MISCH METAL, THORIUM, BISMUTH, SILICON,ZIRCONIUM, TANTALUM, ALUMINUM AND BORON.